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Summary:

Black Holes (BH’s) are curious because of their mysterious nature and unknown properties. BH’s are not empty voids.
Black Holes (BH’s) are curious because of their mysterious nature and unknown properties. BH’s are not empty voids. They are astronomical objects with a gravitational pull so strong that nothing, not even light, can escape it. The “surface” of a black hole, called the event horizon (EH), defines the boundary where the escape velocity (for any particle) exceeds the speed of light, which is the speed limit of the cosmos. Thus, matter and radiation can fall into the event horizon, but they cannot escape.
BH’s can be divided into two main classes. Stellar mass black holes, that form when a star with more than 20 solar masses comes to the end of its life. Another type being Super Massive Black Holes (SMBH’s) believed to exist in the nucleus of galaxies, although their origin is not well understood. There is also the SMBH at the...
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phinds
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Is there a question in there somewhere or did you just feel the need to provide us with a tutorial?
 
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  • #3
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Something went wrong with the SMBH label. It stands for supermassive black hole (the type you find in galaxy centers), not stellar mass black hole.

@phinds: It's an insights article.
 
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vanhees71
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That's a great article (is there no way to give 5 stars to it anymore?).

That brings me to a (maybe stupid) question. Are there galaxies which have no black hole in their center? If not, are there galaxies forming because of the presence of a black hole or the other way around (kind of egg-hen problem)?
 
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  • #5
phinds
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Something went wrong with the SMBH label. It stands for supermassive black hole (the type you find in galaxy centers), not stellar mass black hole.

@phinds: It's an insights article.
Did it start out that way? Seems I would have noticed. The orginal post was VERY long, yes like an article but I didn't think it was posted like an article the way it obviously is now.
 
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Did it start out that way? Seems I would have noticed. The orginal post was VERY long, yes like an article but I didn't think it was posted like an article the way it obviously is now.
You're right, it was migrated into Insights.
 
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Are there galaxies which have no black hole in their center?
M33 appears not to have one.
 
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  • #8
vanhees71
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Intetersting! So a super-massive black hole is not necessary to form a galaxy. There seems to be nothing what doesn't occur somewhere in the universe. Another fascinating thing is that astronomers find more and more dwarf galaxies without dark matter too.
 
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There is a relationship between central black hole mass and the galactic bulge. M33 has no bulge to speak of and no CBH to speak of. There are similarly "thin" galaxies with CBH's, so I think the relationship between these galaxies and their central black holes remains unclear.

The dark-matter less galaxies are still somewhat controversial. The DF2 and DF4 examples are problematic in the same way. There is an interesting paper by Mancera-Pina et al. which purports to have discovered six more. However, the error bars are large, and LSB galaxies suffer from selection biases. I think the "without dark matter" conclusion may be a bit premature - yes, half of the galaxies are on the "zero dark matter" line, but data also fall above and below this line. I'd like to see more data before forming any conclusion.
 
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  • #10
vanhees71
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Very interesting. I find the "DM-free galaxies" so interesting, because it seems to rule out MOND-like theories of gravity, but of course one has to wait for better evidence to draw that conclusion.
 
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Well, I'm not so sure about that. While I don't think MOND has anything to do with gravity (purely my opinion), I think it does tell us something about galaxy formation - not sure what, but something - and that something should be understood. If MOND is telling us something about galaxies, it would explain why it works so well on galactic scales - and nowhere else.

There are other empirical laws, like Tully-Fisher (and Baryonic Tullly-Fisher), and by invoking them one appears wise. For some reason, considering MOND in the same category is considered crazy. Even though BTFR is a prediction of MOND, i.e. the same observed fact can be described in two ways mathematically. Loving Tully-Fisher but hating MOND (as an empirical fact, not as a theory of gravity) is not really a consistent position, but it seems lots of people hold it.

Now onto LSB galaxies. In my best Indiana Jones "snakes" voice, "Why did it have to be LSB galaxies?" They are dim - it's in the name after all - and because they are dim they are hard to see and harder to measure. When you do see one and measure it well, it is likely brighter than average, because otherwise it wouldn't have ended up in your sample. These are among the hardest of measurements to do well, and the thing you would most like in the case of difficult measurements - high statistics - isn't here yet. Rather than pointing at individual outlier galaxies, it would be much, much better to have a distribution of them. We're not there yet.

You should also be careful what you ask for with "DM-free galaxies". How did they get this way? Presumably, they had gravitational interactions with other galaxies that did this, but universality of free fall makes it hard to do. It is especially hard to do without disrupting the baryonic matter in the galaxy. Then again, maybe it's bias: a lot of disruption blows the galaxy apart making it even lower surface brightness, so we don't see it. Maybe a little disruption increases star formation so it's easier to see. Again, we don't know how dark matter stripping is supposed to work, so we don't really know if these galaxies look like they are supposed to after a dark-matter-ectomy.
 
  • #12
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Well, I'm not so sure about that. While I don't think MOND has anything to do with gravity (purely my opinion), I think it does tell us something about galaxy formation - not sure what, but something - and that something should be understood. If MOND is telling us something about galaxies, it would explain why it works so well on galactic scales - and nowhere else.

There are other empirical laws, like Tully-Fisher (and Baryonic Tullly-Fisher), and by invoking them one appears wise. For some reason, considering MOND in the same category is considered crazy. Even though BTFR is a prediction of MOND, i.e. the same observed fact can be described in two ways mathematically. Loving Tully-Fisher but hating MOND (as an empirical fact, not as a theory of gravity) is not really a consistent position, but it seems lots of people hold it.

Now onto LSB galaxies. In my best Indiana Jones "snakes" voice, "Why did it have to be LSB galaxies?" They are dim - it's in the name after all - and because they are dim they are hard to see and harder to measure. When you do see one and measure it well, it is likely brighter than average, because otherwise it wouldn't have ended up in your sample. These are among the hardest of measurements to do well, and the thing you would most like in the case of difficult measurements - high statistics - isn't here yet. Rather than pointing at individual outlier galaxies, it would be much, much better to have a distribution of them. We're not there yet.

You should also be careful what you ask for with "DM-free galaxies". How did they get this way? Presumably, they had gravitational interactions with other galaxies that did this, but universality of free fall makes it hard to do. It is especially hard to do without disrupting the baryonic matter in the galaxy. Then again, maybe it's bias: a lot of disruption blows the galaxy apart making it even lower surface brightness, so we don't see it. Maybe a little disruption increases star formation so it's easier to see. Again, we don't know how dark matter stripping is supposed to work, so we don't really know if these galaxies look like they are supposed to after a dark-matter-ectomy.
Well said regarding MOND being an empirical observation(which it is)
One interesting hypothesis regarding MOND predictions given they fit well for isolated or low density galaxy groups but poorly for clusters or matter distributions in the Early universe is that the equations for MOND apparently share similarities with the equations of state for a superfluid and or a Bose Einstein Condensate. Under this model the distinction could actually represent a phase transition that occurs when a gaseous phase "dark matter" cools down below some critical value. Now I don't know much about these more exotic phase transitions but the prospects seem more believable than a pure replacement of gravity which has been constrained by the near simultaneous Gravitational Wave and Gamma Ray Burst observations due to the deviation from a r^-2 dependence requiring gravitational waves to travel slower than light

If true then if you could construct rotation curves back through time into the early universe there should be a point where MOND begins to deviate from observations for relatively isolated galaxies which would allow you to indirectly confirm the existence of dark matter being a type of matter/substance able to undergo a phase transition. It would be a hell of a lot of work but the prospects are interesting and could contribute to other areas of high red shift astrophysics.

Now at GMU(George Mason University) some of the grad students at Dr. Satyapal's AGN research group involves the study of Supermassive Black Holes in low mass galaxies and or Intermediate Mass Black Holes and one interesting observation I have heard from them is that in lower mass galaxies those within the Mass range of ~10^11 Msun like the LMC or M33 in the local group when "AGN" activity is found surprisingly it is rarely in the center of the galaxy instead they appear to be somewhat random in their distribution. One thought on why this might be the case is that models at least for the direct collapse model of SMBH or IMBH's respectively is that during galaxy mergers they can easily be dislodged from the center with lower mass examples not having the amount of interactions with matter to "drag" them back to the center of the galaxy. If this is true, which is really hard to test since these displaced black holes are not usually going to be in a dense enough environment to accrete much material, then on average every ~10^11 Msun galaxy should have roughly one such black hole somewhere in the galaxies disk halo or core. More massive galaxies on the order of ~10^12 like the Milky Way or Andromeda might then be expected to have around 10 or so black holes of the IMBH to SMBH somewhere within the disk and or Halo if they haven't fallen into the galactic core which might explain why some similar mass galaxies have such different mass central Black Hole and or Bulge. It is an interesting idea though it would be really quite hard to test. There are a few prospective candidates for such Black holes in the Milky Way in a hyper velocity star ejected from the Milky Way's disk far from the center at the sorts of high velocities usually only seen around very massive Black Holes(but that is fairly weak evidence on its own) and there is evidence via both astrometry and a compact radio source in orbit of Sagittarius A* consistent with a Intermediate Mass Black Hole. (Though the latter is a bit contentious after initially proposed subsequent work seems to support its existence via bound orbits within SagA*'s tidal disruption radius with suggest a central mass ~10^4Msun contained within a region no bigger than 400AU which if true is on the small side for a IMBH)

It is an active field of study so we will probably have surprises but we do have an increasing sample size of ~10^11 Msun mass galaxies with displaced central black holes which either way suggests things get far more complicated at the lower mass galaxy range. On fairly extreme example is They have found a distant Dwarf galaxy with a SMBH roughly half the mass of SagA*.
 

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